Making depth migration work in complex structures

Abstract

Velocity variations that exist in areas of complex structure significantly distort seismic wave propagation. TO properly image reflectors in complex structure, these effects from the velocity variations must be accounted for. time migration doesn’t work since it assumes a locally invariant velocity. Depth migration is needed since it can handle the complicated wavefield effects of non-constant local velocity. Depth migration in complex structure using the correct velocity field enables the image to be optimally stacked and positioned at the correct depth. Although the field of depth migration is advancing at a fast pace, it has limited impact in the search for oil because it rarely produces images or depth structures superior to those of conventional methods. This failure results from the difficulty of determining the velocity field for the depth migration. A form of depth migration velocity analysis is needed to produce an accurate interval velocity field. Migration velocity analysis extends many of the concepts of stacking semblance analysis to complex structure. Like stacking semblance analysis, migration velocity analysis will be a key interpretive step in the processing flow. Unlike stacking semblance analysis, the considerable geologic knowledge of the interpreter can significantly help migration velocity analysis. With the increasing speed of computers and the development of interactive systems, the processor or interpreter will soon have the capability to perform migration velocity analysis in his office. Depth migration velocity analysis is the key to making depth migration work better on complex structures and help us find previously hidden